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Design Notes on Exporting U-Boot Functions to Standalone Applications: ====================================================================== |
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1. The functions are exported by U-Boot via a jump table. The jump table is allocated and initialized in the jumptable_init() routine (common/exports.c). Other routines may also modify the jump table, however. The jump table can be accessed as the 'jt' field of the 'global_data' structure. The slot numbers for the jump table are defined in the <include/exports.h> header. E.g., to substitute the malloc() and free() functions that will be available to standalone applications, one should do the following: DECLARE_GLOBAL_DATA_PTR; gd->jt[XF_malloc] = my_malloc; gd->jt[XF_free] = my_free; Note that the pointers to the functions all have 'void *' type and thus the compiler cannot perform type checks on these assignments. 2. The pointer to the jump table is passed to the application in a |
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machine-dependent way. PowerPC, ARM, MIPS, Blackfin and Nios II architectures use a dedicated register to hold the pointer to the 'global_data' structure: r2 on PowerPC, r8 on ARM, k0 on MIPS, P3 on Blackfin and gp on Nios II. The x86 architecture does not use such a register; instead, the pointer to the 'global_data' structure is passed as 'argv[-1]' pointer. |
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The application can access the 'global_data' structure in the same way as U-Boot does: DECLARE_GLOBAL_DATA_PTR; printf("U-Boot relocation offset: %x ", gd->reloc_off); 3. The application should call the app_startup() function before any call to the exported functions. Also, implementor of the application may want to check the version of the ABI provided by U-Boot. To facilitate this, a get_version() function is exported that returns the ABI version of the running U-Boot. I.e., a typical application startup may look like this: |
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int my_app (int argc, char * const argv[]) |
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{ app_startup (argv); if (get_version () != XF_VERSION) return 1; } 4. The default load and start addresses of the applications are as follows: |
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Load address Start address x86 0x00040000 0x00040000 PowerPC 0x00040000 0x00040004 ARM 0x0c100000 0x0c100000 MIPS 0x80200000 0x80200000 Blackfin 0x00001000 0x00001000 |
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NDS32 0x00300000 0x00300000 |
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Nios II 0x02000000 0x02000000 |
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For example, the "hello world" application may be loaded and executed on a PowerPC board with the following commands: => tftp 0x40000 hello_world.bin => go 0x40004 |
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5. To export some additional function foobar(), the following steps should be undertaken: |
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- Append the following line at the end of the include/_exports.h file: |
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EXPORT_FUNC(foobar) |
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- Add the prototype for this function to the include/exports.h file: |
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void foobar(void); |
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- Add the initialization of the jump table slot wherever appropriate (most likely, to the jumptable_init() function): |
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gd->jt[XF_foobar] = foobar; |
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- Increase the XF_VERSION value by one in the include/exports.h file |
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6. The code for exporting the U-Boot functions to applications is mostly machine-independent. The only places written in assembly language are stub functions that perform the jump through the jump table. That said, to port this code to a new architecture, the only thing to be provided is the code in the examples/stubs.c file. If this architecture, however, uses some uncommon method of passing the 'global_data' pointer (like x86 does), one should add the respective code to the app_startup() function in that file. |
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Note that these functions may only use call-clobbered registers; those registers that are used to pass the function's arguments, the stack contents and the return address should be left intact. |